There are several well known types of conventional analog-to-digital converters that have been in use or proposed for various applications. These include types utilizing techniques known as dual slope integration, parallel comparison, voltage to frequency conversion, successive approximation, and serial cascade, for example. Each of these types offers certain advantages for different types of applications and construction. They also have certain inherent disadvantages which render them infeasible or uneconomical for certain uses. For example, one type may be unreliable, expensive to build, or may be too slow in its operation. The disadvantage of such conventional analog-to-digital converters when used for signal processing applications where signal frequency content is important, is the necessity of utilizing well known sample and hold circuits in order to establish a definitive sample-to-sample relationship with a known precise time between samples. Such conventional converters without the sample and hold circuit provide a digital output, which may correspond to a value equal in magnitude to the value of a signal that existed some time between the start of the conversion and the completion of the conversion. Also, in order to be acceptable for signal processing applications, conventional analog-to-digital converters provide complications to a greater or lesser extent when attempting to fabricate them by integrated circuit techniques.
The term "bucket brigade" has been used in the art to describe a type of circuit, where sampled values of a signal are stored in the form of charges in a series of capacitors. Between each of the storage capacitors is a type of switch that is controlled by a clock pulse to transfer the charge from one capacitor to the next, as buckets of water are passed along the line in old methods of fire fighting. Various bucket brigade circuits and their theory of operation is described in Vol. 31, No. 4 of Phillips Technical Review, published in 1970 by F. L. J. Sangster and entitled the "Bucket Brigade Delay Line", a shift register for analogue signals.
Recently, bucket brigade charge coupled circuits have been applied to electrical signal filtering applications. A digital bucket brigade passes a binary signal, a high or a low level, serially through discrete stations or stages with the signal at a station or stage "N" moving to stage "N + 1" in response to each application of some externally applied clock pulse. An analog bucket brigade operates as the digital, except an analog level rather than a high or low level is passed down the line on each application of the clock pulse. In either case, the bucket brigade circuit is an arrangement of components that transfers a given amount of charge from one stage to the next on each application of the external clock; and holds that charge in some form of memory until the next application of the clock. An analogue shift register which operates on the principal of electric charge transfer is described in detail in U.S. Pat. No. 3,643,106 to Berwin et al issued on Feb. 15, 1972.
It is well known that charge coupled circuitry may be readily implemented using low cost integrated circuit techniques. An article entitled "The A, B, C's of CCD's" by Walter F. Kosonocky and Donald J. Sauer published in Electronic Design VIII on Apr. 12, 1975 describes in detail the various methods of fabricating charge coupled devices. U.S. Pat. No. 3,877,056 issued on Apr. 8, 1975 to Bailey points out the advantages and utility of charge transfer devices for signal processing systems. A detailed discussion of charge coupled device technology and its particular applications, is also described in a paper by G. F. Amelio entitled "Physics and Applications of Charge Coupled Devices", in the 1973 IEEE INTERCON Technical Program Papers, Session 6, Paper 1-3.
Although, certain applications of bucket brigage circuits and charge coupled devices are well known, analog-to-digital converters which can be readily fabricated as an integrated circuit, and which eliminates the necessity of sample and hold circuits for signal processing application where signal frequency content is important utilizing such techniques, have not been disclosed, as far as is known.